a b s t r a c tAdvances in the synthetic control of surface nanostructures could improve the activity, selectivity and stability of heterogeneous catalysts. Here, we present a technique for the controlled deposition of TiO 2 overcoats based on non-hydrolytic sol-gel chemistry. Continuous injection of Ti( i PrO) 4 and TiCl 4 mixtures led to the formation of conformal TiO 2 overcoats with a growth rate of 0.4 nm/injected monolayer on several materials including high surface area SBA-15. Deposition of TiO 2 on SBA-15 generated mediumstrength Lewis acid sites, which catalyzed 1-phenylethanol dehydration at high selectivities and decreased deactivation rates compared to typically used HZSM-5. When supported metal nanoparticles were similarly overcoated, the intimate contact between the metal and acid sites at the supportovercoat interface significantly increased propylcyclohexane selectivity during the deoxygenation of lignin-derived propyl guaiacol (89% at 90% conversion compared to 30% for the uncoated catalyst). For both materials, the surface reactivity could be tuned with the overcoat thickness.

We performed a steady state high-yielding depolymerization of soluble acetal-stabilized lignin in flow, which offered a window into challenges and opportunities that will be faced when continuously processing this feedstock.

Catalyst overcoating is an emerging approach to engineer surface functionalities on supported metal catalyst and improve catalyst selectivity and durability. Alumina deposition on high surface area material by sol-gel chemistry is traditionally difficult to control due to the fast hydrolysis kinetics of aluminum-alkoxide precursors. Here, sol-gel chemistry methods are adapted to slow down these kinetics and deposit nanometer-scale alumina overcoats. The alumina overcoats are comparable in conformality and thickness control to overcoats prepared by atomic layer deposition even on high surface area substrates. The strategy relies on regulating the hydrolysis/condensation kinetics of Al( BuO) by either adding a chelating agent or using nonhydrolytic sol-gel chemistry. These two approaches produce overcoats with similar chemical properties but distinct physical textures. With chelation chemistry, a mild method compatible with supported base metal catalysts, a conformal yet porous overcoat leads to a highly sintering-resistant Cu catalyst for liquid-phase furfural hydrogenation. With the nonhydrolytic sol-gel route, a denser Al O overcoat can be deposited to create a high density of Lewis acid-metal interface sites over Pt on mesoporous silica. The resulting material has a substantially increased hydrodeoxygenation activity for the conversion of lignin-derived 4-propylguaiacol into propylcyclohexane with up to 87% selectivity.

Tailoring the interfacial sites between metals and metal oxides can be an essential tool in designing heterogeneous catalysts. These interfacial sites play a vital role in many renewable applications, for instance, catalytic CO2 reduction. Postsynthesis deposition of metal oxide on supported metal catalysts can not only create such interfacial sites but also prevent particle sintering at high temperature. Here, we report a sol–gel-based strategy to synthesize an atomically dispersed “precatalyst”. In contrast to the deposition on catalysts containing preformed nanoparticles, overcoating this material before reductive treatment can inhibit particle growth during thermal activation steps, yielding highly accessible, sintering-resistant Pd clusters that are less than 2 nm in diameter. This synthetic approach allows us to engineer interfacial sites while maintaining high metal accessibility, which was difficult to achieve in previous overcoated materials. Notably, engineering the Pd–ZrO2 interface into an inverted interface with an amorphous ZrO2 overcoat might facilitate a C–O cleavage route instead of a mechanism containing a bicarbonate intermediate during CO2 hydrogenation. We also observed that carbon deposition occurring on methanation sites could be a key factor for improving CO selectivity. Alteration of the reaction pathway, along with the deactivation of certain sites, led to 100% CO selectivity on the ZrO2@Pd/ZrO2 catalyst. This work demonstrated that overcoated materials could represent a promising class of heterogeneous catalysts for selective CO2 conversion over noble metals, which have higher rates and thermal stability compared to state-of-the-art Cu-based catalysts.